10+K entries! No wonder you've been looking for this kind of functionality. :-)

Unless IE does something innately weird when a select element's options exceed 9168 (a stress point, I would imagine), I can think of a couple things to try.

First, experiment with longer values for the delay property of the typeAheadInfo object constructor function.

It's also possible that with so many options, the scripting bogs down with repeated fetches of the options for each keystroke. Perhaps rework the code to perform the retrieval just once (at onload time) and preserve the list in a global variable for faster subsequent retrievals.